Star Delta Contactors Got Burnt When Starting the Motor but The Main Contactor Did Not 9

[imannul98]

Hello

I have a 45 kw induction motor with starting method using star delta method. The star and delta contactors of the motor have been for several occasions got burnt. However, I haven't figured out why the contactors got burnt. The FLA is 93 A, and the contactors have a rating current of 105 A, breaking current of 930 A, and making current of 1050 A. I figured that it was probably caused by the star and delta contactors being closed at the same time during the transition. Hence, I added 1.6 seconds after the star contactor opened for the delta contactor to close. However, it got burnt again.

And then I realized that because the current that flows through the star and delta contactor also flows through the main contactor, thus, if the star and delta contactors got welded, then there should be three contactors that got burnt, not just 2. Does anyone know what seems to be the problem that causes the star and delta contactors to get burnt but the main contactor doe not? Thank You

P. S.
The timer for the star contactor in the close position is 4 seconds.

 

[RRaghunath]

Did you check the speed when the star contactor gets command to open! It could be that the contactor tried to break motor starting current. You indicated breaking current rating for the contactor as 930A, are you sure this is at 0.2 pf (which will be the case if the motor is still running-up)
What is the motor starting current? If the motor happens to be High Efficiency motor, the starting current could be 10--12times the full load current!
If you hook up ammeters during the starting process to measure current during transition, scenario will be clear hopefully.

 

[waross]

Poor connections of the cables into the cable lugs may be causing the problem.
The current rating may be based on a resistive load and the HP rating may be considerably less.
Check the HP rating of the contactors.
If the transition timing is off, the delta contactor may be closing out of phase and the resulting current transient on the delta contactor may be much greater than the starting current.
From the information given, first guess; the contactors are undersized and the transition timing may be off.
The star and delta contactors should be mechanically interlocked and not able to close together.
Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

I have seen repeated failures of contactors when the motor power rating is too close to the source rating (our case is always small generators starting big motors). During the motor start the voltage drop is so large that it causes the magnetic contactors to open and reclose rapidly, repeatedly (chatter). This will lead to very short contact life.

 

[che12345]

Dear Mr. imannul98
My humble opinion for your consideration:
1. A SD starter assembly, the (Main contactor) is (usually) sized [higher rating] than the Star = Delta . This may explain why the Main does not burn out. The fuse or the breaker shall trip, should any of the Star or Delta contactor is welded.
2. Check the (contactors sizes) are based on [AC-3 rating], see IEC for definition of AC-3.
2.1 Check that the three contactors are based on the manufacturer's recommendation for AC-3 operation. Refrain from using contactors of different manufactures.
3. Check that the (contactor coil voltage) which shall be minimum 80% of rated [during starting and running].
3.1 Check the (control transformer) [VA size/rating], if there is one.
4. Check the (starting time). It (may not) [exceed say "approximately" 8s]. If the starting current failed to drop within say approx 8s, the motor could be under-sized or the motor torque produced in
Star is insufficient to overcome the load torque. This may result to the Star contactor burning.
4.1 Check the (three-phase mains voltage) during starting and running. Voltage may drop to 70% during starting and at 95% during running.
4.2 Check the motor starting and running current.
5. Check that the (Delta contactor) is [AC-3 rated], see above 2. An AC-2 rated contactor may burn-out, under prolong running.

Che Kuan Yau (Singapore)

 

[imannul98]

RRaghunath

Yes, I checked the starting current, which only about 165 A when the load is light. The speed when the star is open is about 800 rpm of full speed 1000 rpm. I haven't checked the current yet if the load is heavy because the management currently doesn't allow it(The contactors got burnt when the motor is under a heavy load). However, if the contactors got burnt because the current is too high, shouldn't the main contactor got burnt too? the main contactor and the star and delta contactor have the same rating.

 

[TugboatEng]

The star contact should be open during heavily loaded operation. How did it burn under heavy load?

 

[RRaghunath]

800rpm is 80% of rated speed approximately. That's too early for transition from star to delta, as the motor would have been still drawing starting current. This meàns, star contactor is trying to break 165A inductive current for which it is not rated.
I have a concern with 165A starting current you mentioned. This seems low even in star mode. I am wondering whether you have excessive voltage drop in cables to motor or whether your source is so weak to cause much voltage drop at the busbars.
What is the rated starting current as per motor name plate?
What is the MCC bus / incomer voltage while motor is being started.
If the source voltage is dropping, it is more serious as it affects the contactor pickup.

 

[imannul98]

che12345

Dear Che,

the main contactor has the same rating with the other two contactors, and it's also AC-3 rated. However, I don't understand about the contactor's coil voltage. Is it different with the main voltage? The contactor I mentioned is this.

 

[che12345]

Dear Mr. imannul98
Follow up No.1
1. According to your data: motor Voltage ??, 45kW, 93A, 1000rpm, Voltage not stated, may be 380V?.
On light load: transfer time time 4s at (800rpm). Istart (165A). All three contactors are of equal size.
2. Transfer at 800rpm and Istart 165A seems to be low. But, they are unlikely? to be the culprit.
2.1 The burn-out could? be when starting with heavy load.
Proposal: a) On heavy starting, transfer when the motor had reached say approx. 900rpm, while the current should had dropped to approx. 140A.
b) If the rpm is unable to reach say approx. 900rpm or the current remains high, the motor is likely? under-sized.
3. The (contactor coil voltage) is usually 110V, 230V through a [local step-down control transformer]; but, very unlikely 380V = the mains voltage. FYI: the contactors can be fitted with any preferred coil voltage via a local step-down control transformer.
4. Observation:
a) If the contacts burn-out is by (over-heating), it is likely due to [over-loading by heavy current for prolong time].
b) If the contacts burn-out is with a (heavily splash), it is likely due to [shorting] (e.g. Delta closes before the Star arc is fully quenched. Noted: You had added a 1.6s timer is fine.
5. Attention: The conductor (insulation) seems to be [over-heated] in 2/T1 and 4/T2 terminals. Check the tightening torque or use cable lug as used in L1,2 &3.
Che Kuan Yau (Singapore)

 

[itsmoked]

About the coil voltage. It can be confusing because they never list the coil voltage on the main name plate as its information always stays the same regardless of the coil voltage.

The coil voltage will be listed on the actual coil. Search for where the coil is connected to the field wiring and in the immediate vicinity you will see the coil voltage listed. It's on the side of the contactor not usually the face.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

You say with a light load the transition occurs at 80% speed. That would mean with a heavier load that the transition will occur at an even lower speed.

Then, you say there is a 1.6s open circuit time delay in the transition. That will let the motor decelerate significantly before power is applied again.

Both of the above combined say that you are using the wrong starting method.

The transition should occur at >95% motor speed and the motor speed needs to remain high until the transition is complete. Try a longer time before the transition and see if the motor speed keeps climbing. If the motor speed hangs up and it stops accelerating then you are definitely using the wrong starting method.

Overall, star-delta starting is hard on the contactors. Most times, the contactors chosen are the minimum rated for the application. That will generally cause the short contactor life you are seeing.

 

[che12345]

Dear Mr. imannul98
Follow-up No. 2
It would be helpful if you can attach the following:
a) the (control schematic) including the [power line connections and the control circuitry].
b) the image of the (burn-out moving and the fixed contacts), showing the damage is [by over-heating or
by short-circuiting].
c) the (additional of 1.6s dwelling time) between opening of the Star before closing of the Delta may be lowered to 0.1s. Or use the [specific Star-delta timer] recommended by the manufacturer.
Note:1. the (specific Star-delta timer) has an internal non-adjustable dwelling time of around 50ms.
2. the addition of the dwelling time of 1.6s (which may be reduced to 0.1s) is unlikely? to be the culprit.
Che Kuan Yau (Singapore)

 

[edison123]

From your photo, terminals 1, 3 & 5 seem to be properly lugged and 2, 4 & 6 are just bunched up wires? That could explain the overheating and lower motor voltage due to insufficient contact area at 2, 4 & 6.

Muthu

http://www.edison.co.in

 

[waross]

One the one hand I have seen thousands of cables directly connected like that with no problems.
On the other hand that looks like seven strand cable and seven strand about that size will loosen easily if the cable is not "rolled in" when it is installed.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

Now that everybody is talking a out the picture, I zoomed in and there is quite a lot of dust in that enclosure. That can't be good for the contacts, either.

https://ieeexplore.ieee.org/document/4318186

 

[LionelHutz]

The cause of failures is likely rather simple. The contactors are simply too small for the application. The contactors are sized per AC3. The starting method has the contactors breaking motor starting level currents and to break motor starting currents requires AC4 rated contactors.

Overall, the starting method is simply wrong because it should only be used on applications where the motor gets to >95% speed before the transition happens.

Also, closed transition with another contactor and resistor is the best way to built a Y-Delta starter.

 

[waross]

I agree with you Lionel. lps
I will add, the delta contactor may be closing out of phase with resulting currents greater than the maximum starting current.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

At this point just replace the mess with a soft starter.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[imannul98]

Dear Lionel

Could you please explain a bit about the 95% speed. Is it 95% of rated speed or 95% of any speed of the motor?(Because most of the time, the motor started at low speed, about 200 or 300 rpm from rated 1000 rpm, we change the speed by changing the motor tap). Also, could you give me references or circuit diagram about the contactor-resistor starting in star delta method. Thanks